This invention relates to a powdery instant milk composition of improved quality and a method of producing the composition.
Since powdery milk such as ordinary powdery skim milk or whole milk powder is low in solubility in water, a powdery instant milk (or a quickly dissolving powdery milk) has been produced by special methods.
Two factors are involved in the solubility of powdery milk; namely, the solubility is determined by the physical properties of the powder (first factor) and crystallization of lactose contained in the powdery milk (second factor). In general, powdery milk is in the form of a dense powder of single grain structure. The capillary space within the powder is very small. If put in water, the contact portion of the powder with water is rapidly made wet to form a highly viscous sirup layer, with the result that the capillary space leading into the inner region of the powdery means is plugged to prevent water from permeating into the powdery mass. It follows that the powdery milk forms lumps and is not dissolved in water. In contrast, a quickly dissolving powdery milk has a crumb structure having a large capillary space within the powdery mass. Thus, water promptly permeates into the powdery mass, allowing the powdery milk to dissolve in water without forming undissolved powder lumps.
Crystallization of lactose (second factor) is also important. The lactose contained in the ordinary powdery milk is amorphous and so high in moisture-absorbing property that the powdery milk, if put in water, promptly absorbs water to form a sirup layer. As described previously, the sirup layer causes formation of undissolved powder lumps. In contrast, a large portion of lactose contained in the quickly dissolving powdery milk is crystallized. The crystallization brings about capillary space network formed of cracks extending along edges and side surfaces of very small crystals, with the result that water promptly permeates into the powdery mass. It follows that a sirup layer is not formed on the powdery mass surface and the powdery milk is promptly dissolved in water. The crystalline lactose contained in the quickly dissolving powdery milk has two optical isomers, i.e., .alpha.-lactose and .beta.-lactose. It is reported by many researchers that the .alpha.-lactose molecule is combined with one molecule of water of crystallization to form the lactose crystal.
It was customary to produce the quickly dissolving powdery milk by special spray drying method or freeze drying method. But, the conventional method gives rise to the defects described in the following. First of all, any of the conventional methods requires large facilities and large quantities of energy, leading to a high manufacturing cost of the quickly dissolving powdery milk, compared with the ordinary powdery milk. Secondly, drying is performed in two stages in the special spray drying method to promote conversion of the powder particles into crumb structure and to facilitate crystallization of lactose. The two-stage drying makes the total heating time of the milk components considerably longer than the heating time in the ordinary spray drying method. In addition, water is evaporated in the second stage drying step at considerably high temperatures (more than 100.degree. C.), with the result that the milk components tend to be thermally denaturalized. A third defect is that the quickly dissolving powdery milk produced by the conventional freeze drying method cannot be preserved satisfactorily for a long period of time. Specifically, major portion of the fats and oils contained in the ordinary powdery milk is coated with milk proteins such as casein. However, major portion of the fats and oils contained in the quickly dissolving powdery milk produced by the freeze drying method is not coated, but is exposed to the atmosphere, with the result that the fats and oils are rapidly spoiled, leading to a low preservation capability mentioned above.
Powdery milk is used for producing a substitutive powdery milk composition which is used in place of or in combination with whole milk as feedstock of calves. The substitutive milk composition is powdery during transportation and storage, and is dissolved in cool or warm water when fed to calves. Thus, it is desirable for the substitutive milk composition to dissolve in water easily.
A quickly dissolving substitutive milk composition may be produced by using as a main component the quickly dissolving powdery milk converted from powdery skim milk. Specifically, the composition may be produced by mixing with the main component other components such as dry whey, animal fats, and livestock feed having fish solubles adsorbed thereon. Of course, the other components should desirably be readily soluble in water, also. It is also possible to produce the quickly dissolving substitute milk composition by the method equal to the method of producing the quickly dissolving powdery milk, using a solution containing both powdery skim milk and the other components. However, the conventional method of producing a quickly dissolving powdery milk is employed in each of the two cases described above, with the result that the three defects mentioned previously, i.e., high manufacturing cost, thermal denaturalization of milk components, and spoiling of fats and oils, remain unsolved. Moreover, the defects of manufacturing cost and thermal denaturalization are more serious in the substitutive milk composition.
Specifically, the high manufacturing cost of the substitutive milk composition gives serious economic damage to the livestock breeder. On the other hand, thermal denaturalization of the milk components is quite undesirable in nutritive and physiological aspect of the calf. Specifically, casein contained in the powdery milk is known to be coagulated under the action of rennin, i.e., a digestive enzyme, within the fourth stomach of a calf before the ruminating stage to form a so-called "curd". The curd is gradually transferred into the small intestines to be digested and absorbed. The time to form the curd within the fourth stomach and hardness of the formed curd are known to be particularly important in nutritive and physiological aspect of the calf. It is also known that the curd-forming ability of the powdery milk depends on the quality of casein contained in the powdery milk. If a powdery milk is heated to temperatures above 75.degree. C., casein contained therein is known to be denaturalized, leading to a low curd-forming ability. What should be noted is that, if a normal curd is not formed, the substitutive milk composition is promptly transferred from the fourth stomach into the small intestines. Naturally, the small intestines are caused to bear an excessive load, leading to scours and abortion of the calf. To be brief, various difficulties accompany to the production of a quickly dissolving substitutive milk composition.